Process and apparatus for the crystallization of a melt

ABSTRACT

A melt is crystallized by introducing the melt into a gap formed between two belts. The gap-forming flights of the belts travel in opposite directions. An upper one of the flights is cooled so that an upper surface of the melt crystallizes thereon and is removed from the gap on the upper belt. The lower belt removes the residual melt from the gap and that residual melt is recycled to the gap.

This application is a continuation of application Ser. No. 07/804,599,filed Dec. 10, 1991, now abandoned.

BACKGROUND OF THE INVENTION

The invention concerns a process and an apparatus for thecrystallization of melts, wherein at least one wall coolable to atemperature below the crystallization point of the melt is provided, thewall being in contact with the melt so that crystallization takes placeon it.

Processes and apparatus are known (e.g., see Swiss Patent 501,421)wherein crystals are formed by the extraction of heat from the melt of achemical product. In the known processes and apparatuses, tubularcrystallizers are used, wherein a melt and a coolant run down alonginternal and external walls thereof, respectively, in the form oftrickling films, so that the desired crystallization takes place. Thecrystallization is induced for the purpose of solidifying a melt orpurifying a chemical product.

The purifying process is based upon the fact that crystals are purerthan the initial melt. Thus, by carrying out the above-describedcrystallizing process, impurities can be pressured against the grainboundaries upon the formation of crystals, so that the impurities remainin the residual melt. The known processes and configurations operatediscontinuously. Initially, the melt is caused to crystallize on thecooled walls of the crystallizers. Afterwards, the crystalline layer ismelted and runs down the walls.

It is an object of the present invention to develop a process and anapparatus of the above-mentioned type so that it is no longer necessaryto work discontinuously, but rather so that a continuous recovery of thecrystallized product is possible.

SUMMARY OF THE INVENTION

To attain this object, initially in a process of the afore-mentionedtype two walls moving in opposite directions are provided one of whichwalls is supplied with the melt, while the other wall is a cooled wallbrought into contact with a surface of the melt. That surface of themelt crystallizes on that other wall and is continuously drawn offtherewith to be further processed.

Preferably, the cooled wall carrying the melt is at a temperature in thevicinity of the crystallization point, whereby the melt surface incontact with the cooled wall crystallizes very rapidly.

An apparatus according to the invention wherein the walls are formed bytwo endless belts defining a gap between two opposing flights thereof.The flights travel in different directions. The melt is applied to oneflight and travels thereon into the gap. The other flight is cooled andis in contact with an opposing surface of the melt to crystallize thatsurface. The crystallized layer leaves the gap on the cooled belt and isremoved therefrom.

Preferably, the cooled belt constitutes an upper one of the belts, andthe lower belt is heated. A scraper or the like removes residual meltfrom the lower belt. The residual melt can be remelted and re-fed to thegap. The belts can be heated and cooled by sprayers which spray hot orcold liquid thereagainst.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the invention will become apparent fromthe following detailed description of a preferred embodiment thereof inconnection with the accompanying drawings in which like numeralsdesignate like elements, and in which:

The Sole FIGURE shows schematically in side elevation an apparatus forcarrying out the process, according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The apparatus comprises a double belt installation, including a lowerbelt 1 and an upper belt 2, which are guided around reversing rolls 3,3a and 4, 4a. The axles 5 of the reversing rolls 3, 3a, 4, 4a arehorizontal. The belts 1, 2 may comprise steel bands. The lower belt 1 issomewhat longer than the upper belt 2 and its upper flight 1a projectsupstream past the reversing roll 4 of the upper belt 2. Disposed abovethat upstream portion of the flight 1a is a feeder device 6 for feedinga melt 7 produced by the melting of a material capable ofcrystallization. The feeder device 6 may comprise a feeder pipe with aslit in its bottom side extending transversely to the running direction8 of the lower belt 1. However, a known strip caster or means to applythe melt in the form of drops, may also be used.

The upper belt 2 revolves in a direction 9 (i.e., clockwise in theFIGURE) which is the same as that of the lower belt. The flight 1a ofthe lower band and the lower flight 2a of the upper band form a parallelgap therebetween, and those flights travel in opposite directions.

Within areas surrounded by the belts 1, 2, respectively, are disposedspraying devices comprising a plurality of spray nozzles 10 and 11. Thespraying devices are located within a cover 12 and 13 in a manner suchthat the upper flight 1a of the lower belt 1 is sprayed from below, andthe lower flight 2a of the upper belt 2 is sprayed from above. A liquidis supplied to the lower spray nozzles 10 through conduit system 14, anda liquid is supplied to the spray nozzles 1 by means of a conduit system15. Those conduit systems 14, 15 are connected in a manner (not shown indetail) with a fluid circulation system laterally relative to theendless belts.

The liquid supplied to the conduit system 14 is maintained at atemperature in the vicinity of, but preferably slightly higher than, thecrystallization temperature of the melt 7. The conduit system 15, on theother hand, is supplied with a cooling liquid whose temperature issubstantially below the crystallization temperature of the melt 7.

The spraying liquids thus impact against the sides of the respectivebelts 2 and 1 facing away from the gap 16 in which the melt 7 is locatedin order to add heat to, or remove heat from, the melt 7 in the gap.That is, the liquid sprayed against the upper strand 1a of the lowerbelt, maintains at a predetermined temperature the surface of the meltcontacting the flight 1a, while the surface of the melt 7 in contactwith the lower flight 2a of the upper belt 2 is strongly cooled withinthe gap 16 by means of the liquid sprayed from nozzles 11. Thus, on thelower flight 2a of the upper belt beginning in an area upstream of thereversing roll 4a, a crystallization layer 17 will be formed from themelt 7, which layer is moved out of the gap 16 in the travelingdirection 9 of the upper belt 2a, i.e., to the left in the FIGURE,opposite the direction of feeding of the melt. The crystallization layer17, which continues to adhere to the upper belt 2 outside of the gap, isremoved from the belt 2 by means of a take-off deice 18, for example astripping blade and is placed on a conveyor 19 capable of moving thecrystalline layer. The crystalline layer 17 crumbles upon being takenoff the belt and is transported laterally out of the range of therevolving belts 1 and 2 for further processing.

The reversing roll 3a of the lower belt 1 is combined with a strippingdeice 20, for example in the form of a spatula or the like, whereby theresidual melt 7' is removed from the terminal area of the lower belt 1and passed into a collector device 21. From this collector deice 21 themelt enters a recycle line 23 and is transported to the feeder device 6through a heat exchanger 22 which remelts and heats the melt to theinitial feeding temperature. It would also be possible to equip thecollector deice 21 itself with a heat exchanger in order to recycle theresidual melt. The apparatus described with reference to the drawingrenders possible the continuous production of crystals for furtherprocessing.

Although the present invention has been described in connection with apreferred embodiment thereof, it will be appreciated by those skilled inthe art that additions, modifications, substitutions, and deletions notspecifically described may be made without departing from the spirit andscope of the invention as defined in the appended claims.

What is claimed is:
 1. A process for the crystallization of a meltwherein a melt is fed into a gap formed between two walls traveling inopposite directions, one of said walls constituting a crystallizationwall which is cooled to a temperature below the crystallizationtemperature of the melt so that a surface of the melt engaging saidcrystallization wall crystallizes thereon and is removed from said gapon said crystallization wall.
 2. A process according to claim 1, whereinthe other of said walls is maintained at a temperature no lower thansaid crystallization temperature and conducts residual melt from saidgap.
 3. A process according to claim 2, wherein said other wall ismaintained at a temperature above said crystallization temperature.
 4. Aprocess according to claim 1, wherein said crystallization wall travelsabove the other wall, said other wall conveying residual melt from saidgap.
 5. A process according to claim 4, wherein said crystallizationwall travels toward a location where melt is introduced into said gap.6. A process according to claim 1, wherein said walls are formed byfirst and second continuously rotating endless belts.